In a cathode ray tube of high brightness, a reproduced optical image of high brightness is formed by increasing the energy of an electron beam which impinges upon its phosphor screen.
Heat generated in the phosphor screen when an electron beam of high energy impinges on the phosphor screen or in addition thereto heat generated in such a case when the electron beam impinge on an electron beam landing position determining electrode such as a shadow mask, an aperture grille or the like which is disposed in opposing relation to the phosphor screen within a tube envelope so as to restrict the electron beam landing position relative to the phosphor screen becomes considerably great as the energy of the electron beam is increased. However, the front panel or glass panel of the cathode ray tube envelope on which the phosphor screen is formed is low in thermal conductivity so that particularly in the continuous driving of the cathode ray tube, the rise of temperature at the central portion of the glass panel which is difficult to radiate its heat becomes significant. As a result, a so-called thermal quenching takes place in the phosphor. The thermal quenching is such a phenomenon that as the temperature rises, the brightness of the phosphor is lowered. Since the degree of thermal quenching is different depending on the phosphors of respective colors, white balance becomes out of order.
The disorder of the white balance at the center of the front panel much deteriorates the picture quality so that, upon continuous driving of the cathode ray tube, in order to establish the white balance at the center of the front panel, it may be considered to adjust the brightness of the optical images of the respective colors. In this case, there occur such defects that the white balance on the peripheral portion of the front panel is destroyed and that the brightness of the whole portion can not be increased.
Such problem becomes serious in either case of a color projector in which, for example, picture images of respective colors obtained from respective monochromatic cathode ray tubes are projected in mixed state on a screen to produce a color picture image thereon or a color picture image formed of picture images of a plurality of colors is produced from the same cathode ray tube and then projected onto the screen.
Therefore, in the cathode ray tube of this kind, in order to prevent the temperature from rising to such an extent that the thermal quenching takes place on the phosphor screen even upon continuous driving thereof, its front panel must be cooled. This cooling may be carried out by using a cooling fan. In this case, the cooling fan, however, sends not only air but also dusts to the surface of the front panel of the tube envelope. Then, the dusts adhere to the panel surface to cause an apparent deterioration of brightness. In this case, there occurs also a problem of a noise of the cooling fan.
To avoid the above shortcomings, a cathode ray tube apparatus was proposed in which a transparent liquid coolant, particularly a liquid capable of easily causing convection is disposed in contact with the front panel of the tube envelope to cool the front panel.
Such liquid cooling type cathode ray tube apparatus, particularly a cathode ray tube apparatus of a closed convection type is shown in FIG. 1, for example, as a partially cross-sectional side view. Such cathode ray tube apparatus comprises a tube envelope 1, a front panel 1a of flat plate shape on the inner surface of which a phosphor screen 7 is deposited, a flat plate shaped transparent panel 2 having optical transmissivity made of glass and opposed to the front panel and a metal spacer 3 of annular frame shape having excellent thermal conductivity interposed between both the panels 1a and 2. The spacing between the panels 1a and 2 is determined by the metal spacer 3. The frame-shape metal spacer 3, the outer surface of the panel 1a and the inner surface of the panel 2 are bonded one another by a resinous bonding agent, for example, a silicone resin 4 and also are sealed in liquid tight to form a liquid tight space 5 between the panels 2 and 1a. Sealed and filled within this liquid tight space 5 is a transparent liquid coolant 6 which easily causes convection.
Upon use, the tube envelope 1 thus made is so located that its panel 1a is disposed in substantially the vertical direction or inclined obliquely.
In this case, the liquid coolant 6 filled into the closed space 5 directly contacts with the outer surface of the front panel 1a within the tube envelope 1 thereby thermally coupled in tight to the outer surface of the front panel. Therefore, according to such configuration, when the temperature of the panel 1a rises, the liquid coolant 6 is effectively heated by the panel 1a. Then, the liquid coolant 6 thus heated is moved upwards to cause convection within the space 5. Thus, even the heat in, for example, the central portion of the panel 1a is effectively carried to the peripheral portion of the panel and conducted to the metal spacer 3 having excellent thermal conductivity made of, for example, aluminium which is disposed in the above peripheral portion. The heat is then conducted throughout the metal spacer 3 and then radiated from the outer peripheral portion of the metal spacer which is in contact with the open air or a heat radiating path such as a chassis or the like.
According to the cathode ray tube apparatus thus made, the rise of temperature in the panel 1a can be suppressed relatively effectively.
However, recently, in a video projector for example, its cathode ray tube has been requested to have high brightness, high resolution and high power as the brightness is increased, thus the more and more effective heat radiation being requested. If in accordance with the increase of power (power P is given by an equation, P=Vp.times.Ik where Vp is anode voltage (acceleration voltage) and Ik is cathode current), its acceleration voltage is increased, the front panel of the tube envelope 1 must increase its thickness so as to avoid the increase of transmittance amount of X-ray. However, in the video projector, when a lens, particularly a plastic lens is used in its optical system, from a lens designing view point, it is not possible to increase the distance between the phosphor screen 7 and the lens, namely, the thickness of the front panel 1a so much. In this case, it is therefore employed such a method that in the glass material of the transparent panel 1a, the containing amount of, for example, lead which has a shield effect against X-ray is increased. However, such glass containing a large amount of lead is lowered in hardness and becomes a property apt to be scarred easily. Accordingly, in this case, when the temperature rises and a deformation such as bend and the like due to thermal expansion occurs in the transparent panel 2, the transparent panel is particularly broken easily. Therefore, as the brightness is improved to be high, the heat radiation and cooling are requested to become more effective.
For this reason, in the prior art cathode ray tube apparatus thus constructed as, for example, shown in FIG. 1, a heat radiating fin 8, for example, is provided to increase the surface area which contacts with the air. However, the heat radiation is not so effectively carried out by such countermeasure. As a result of various experiments and considerations, the present inventors have clarified the reason for the above defect that the heat of the liquid coolant 6 is not effectively conducted to the metal spacer 3. That is, practically the metal spacer 3 is bonded in liquid tight to the panels 2 and 1a at the both outer and inner surfaces of the portion interposed between both the panels 2 and 1a by the resin 4 so that the area of the metal spacer 3 which contacts with the liquid coolant 6 is small and hence the heat of the liquid coolant 6 is not effectively conducted to the metal spacer 3.
On the basis of this clarification, the present applicant has previously proposed a cathode ray tube apparatus in which heat of a liquid coolant can be effectively conducted to a metal spacer as a Japanese patent application No. 101550/1982. FIG. 2 shows an example of such cathode ray tube apparatus, and in FIG. 2, like parts corresponding to those in FIG. 1 are marked with the same references. In this case, the metal spacer 3 is provided at its inner periphery with an inner peripheral protruded portion 3e of a plate shape which is thinner than other portions. This inner peripheral protruded portion is immersed into the liquid coolant 6 sealed within the space 5 and directly contacts therewith whereby to increase the area in which the metal spacer 3 contacts with the coolant 6.
When the metal spacer 3 is provided at its inner periphery with the protruded portion 3e which is immersed into the liquid coolant 6, the efficiency in which the heat of the liquid coolant 6 is conducted to the metal spacer 3 is increased but this inner peripheral protruded portion 3e must be disposed outside the effective picture screen around the picture screen of the cathode ray tube, thus the area of the inner peripheral protruded portion 3e being restricted.
Further in practice, when the cathode ray tube type projector is constructed, as shown by its schematic cross-sectional view in FIG. 3, a lens system 9 is disposed in opposing relation to the transparent panel 2 of the above cathode ray tube apparatus. This lens system 9 is fixed such that its mirror cylinder 10, for example, is fixed to a lens holder 11 of cylinder shape disposed on the front periphery of the tube envelope 1 by screwing, for example, three attaching leg pieces 12 extended to the outside from the end portion of the mirror cylinder 10. The lens holder 11 is provided at its rear end with a flange portion which is fixed to a chassis 13 together with the metal spacer 3. According to the configuration thus made, while the heat from the tube envelope is directly radiated from the outer periphery of the tube envelope itself, heat is radiated from the metal spacer 3 to the chassis 13 and further heat is radiated to the air from the surfaces on which the metal spacer 3 and the transparent panel 2 contact with the air. Although the metal spacer 3 and the transparent panel 2 are surrounded by the lens system 9 and the lens holder 11, this surrounded space is communicated with the air through the clearance provided between the periphery of the lens mirror cylinder 10 and the lens holder 11. Thus, heat is radiated thereby and heat is radiated also from the lens holder 11 made of metal. However, when the lens system is provided opposing to the cathode ray tube as set forth above, if a bright lens the so-called F number of which is small is used as this lens system, it is desired that the distance between the lens system and the picture image on the cathode ray tube, namely, the spacing between the lens system 9 and the front panel 1a of the tube envelope 1 is made as small as possible, thus the thickness of the metal spacer 3, the thickness of the liquid coolant 6, the thickness of the transparent panel 2 and the like being restricted, respectively. Further, when the temperature of the liquid coolant 6 rises, in order to prevent the panel 2 from being deformed and broken by the thermal expansion of the liquid coolant and to prevent the liquid tight condition of the portion sealed by the resin 4 from being damaged to cause the liquid to escape, it is desired that the volume of the liquid coolant, accordingly, the thickness of the coolant 6 is made small. In accordance therewith, the thickness of the metal spacer 3 is reduced. Accordingly, in order to enhance the cooling effect of the cooling by the liquid of closed convection type, further consideration has to be made. By way of example, when a plastic lens is used as the lens system, the plastic lens having an F number as small as about 1.0 can be prepared. In this case, in the cathode ray tube of 7-inch type, the distance between the lens system 9 and the front panel 1a of the cathode ray tube becomes as, for example, about 20 mm. Further, there is a spatial restriction due to the provision of the lens holder 11. In addition, when cathode ray tubes of red, green and blue colors are arranged just like, for example, a 3-tube type projector, in order to make the whole of the apparatus small, the above spatial restriction becomes more severe. As a result, the increase of the surface area or the like of the metal spacer or the like so as to effectively radiate the heat from the liquid coolant 6 is restricted.
In the liquid cooling closed convection type cathode ray tube which is applied as the cathode ray tube having high brightness for use with the projector of, for example, color cathode ray tube type, this invention is to further improve the heat radiating effect regardless of the above restrictions.